The use of a drilling fluid in drilling operations using the rotary drilling method is well known. The drilling fluid functions to lubricate and cool the drill bit, serves as a carrier the cuttings made by the drilling bit in the borehole, and supplies a hydrostatic head on the formation to counterbalance any pressures of liquids or gasses which are encountered in the various strata through which the drill bit passes. It also creates a plastering effect on the wall of the borehole to limit the flow of fluid from the borehole into the formation, leaving the formation in as near a natural state as possible.
Aqueous drilling fluids are preferred at many sites due to their relatively lower cost compared with oil-based drilling fluids or invert emulsion fluids. Conventionally, aqueous drilling fluids comprise fresh or salt water, clay, weighting agents, and other additives such as thinning agents, fluid loss agents, and the like. One common additive is glycols and polyglycols (polyalkoxylates), which serve to enhance lubrication and reduce shale swelling.
During the course of drilling, the level of glycol and/or polyglycol in the drilling fluid may fluctuate due to diffusion or absorption by the formation. Drilling muds are thus tested at least once daily to monitor the condition of the mud and establish a basis for future recommendations to minimize borehole problems, thus assuring that the basic functions of a mud are carried out as expected. Because of their usefulness in drilling fluids, there is a need for an easy, accurate method for monitoring glycol and/or polyglycol levels on site, that is, in the field. Previous methods for such monitoring include total distillation, which is inconvenient and can lead to inaccuracies if not per/brined with precision, and measurement of the mud's refractive index, which can be inaccurate due to the presence of colored materials and other solutes.
A method for testing polyglycol concentration in the laboratory using complexation with cobaltothiocyanate has been described in "Detergent Analysis, a Handbook for Cost-Effective Quality Control," by B. M. Milwidsky and D. M. Gabriel, published by John Wiley and Sons, New York (1982), p. 100. However, this method requires relatively large amounts of dichloromethane and access to a spectrophotometer, making it impracticable for use at a drilling site.
Accordingly, there remains a need for an efficient, easy and accurate method for monitoring glycol and/or polyglycol levels in drilling muds and other borehole fluid media.